1. Field of the Invention
The invention relates to stilbene derivatives that are useful for the treatment of diseases characterized by cell hyperproliferation. For example, the invention provides new stilbene derivatives including water-soluble pro-drug forms of stilbene derivatives, and contemplates use of the stilbene derivatives for the treatment of human malignancies and non-malignant diseases such as liver cirrhosis.
2. Related Art
Cancer is a deadly disease causing significant morbidity and mortality. Current existing cancer therapeutic agents provide only marginal benefit for treatment of some cancers. Thus, the development of novel agents for cancer therapy is of great interest.
Stilbenes are a group of natural compounds with a wide range of biological activities. The hydroxylated stilbene resveratrol (3,4′,5-trihydroxyl-trans-stilbene) is a phytoalexin present in grapes, and plays a role in the prevention of coronary artery disease associated with red wine consumption [1]. The mechanism is related to suppression of platelet aggregation, alteration of eicosanoid synthesis and modulation of lipid metabolism. Resveratrol also has antioxidant and anti-inflammatory properties and could be a potential chemopreventive agent [17].
In addition, resveratrol has a potential therapeutic effect in suppressing tumor progression [18]. In vitro inhibition of cell proliferation [4] and in vivo anti-neovascularization by resveratrol have been demonstrated [2]. Resveratrol enhances TRAIL-induced apoptosis through G1 cell cycle arrest and depletion of survivin [19]. The apoptotic effect of resveratrol can be overcome by overexpression of Bcl-2 or FADD-DN. However, Bcl-2 or FADD-DN cannot interfere with resveratrol-mediated cell cycle arrest or survivin depletion, indicating that overexpression of Bcl-2 or FADD-DN can separate the effect of resveratrol in the cell cycle and apoptosis [19]. Resveratrol may inhibit cell migration by altering the cytoskeleton, which could be due to induction of tensin [20]. In cultured human breast cancer cell lines, resveratrol induces formation of filopodia and decreases the activity of focal adhesion kinase (FAK) and formation of focal adhesion complexes [21]. With respect to other related compounds, stilbene derivative 3,5,4′-trimethoxy-trans-stilbene induces microtubule disassembly by depolymerization of tubulin in endothelial cells, which leads to inhibition of blood vessel growth and disappearance of pre-existing blood vessels in chick and zebra fish embryos [22]. Another derivative, 3,4,5,4′-tetramethoxystilbene causes rapid appearance of perinuclear aggregation of mitochondria in WI38VA cells and activation of caspases [23], supporting a cytotoxic effect of stilbenes.
Based on the interesting anti-tumor effect of resveratrol and other stilbene derivatives, Roberti et al. synthesized a series of stilbene derivatives in both cis and trans orientations by placing OH, NH2 or OCH3 groups at positions 3′ and 4′ and OCH3 at positions 3,5. The IC50 for each of the stilbene derivatives was tested in HL60 cells. Several active stilbenes were identified, and among them, cis-3,4′,5-trimethoxy-3′-aminostilbene (stilbene 5c; Scheme 1 of Roberti et al. J. Med. Chem. 46:3546-3554 (2003)) and cis-3,4′,5-trimethoxy-3′-hydroxystilbene (stilbene 6c; Scheme 1 of Roberti et al. J. Med. Chem. 46:3546-3554 (2003)) were the two most active compounds that induced HL60 apoptosis in nanomolar concentrations (IC50=30 nM) [5]. More interestingly, the cytotoxicity of these two stilbenes was not affected by overexpression of multiple-drug resistant (MDR) gene [5], a gene that is responsible for drug resistance in leukemia and many other cancers. This study illustrates the potential of using stilbene derivatives for treatment of cancer. However, the mechanism of stilbene-induced cell death remains unknown.
Microtubules play an important role in molecular transport in cells and form mitotic spindles that are essential to segregation of chromosomes. Microtubule-interfering agents have a high potency in suppressing cell proliferation [7]. The major component of microtubules is tubulin, which contains three different sites for potential drug targeting: the colchicine binding site, the vinca alkaloid binding site and the taxane binding site. The existing chemotherapeutic agents, vincristine and paclitaxel, are compounds that specifically target vinca alkaloid binding site and the taxane-binding site, respectively, and trigger cancer cell death [32]. However, compounds targeting the colchicine site are not yet available for clinical applications, although they have been pursued by many investigators [6, 7].
Tumor growth requires the development of a network of neovasculature to supply oxygen and nutrients and to remove toxic metabolites. The neovasculature formed in the tumor tissue differs significantly from normal vasculature [24, 25]. Targeting the tumor vasculature has evolved into a useful strategy to develop new cancer therapeutics [26]. Two approaches are currently used. One is to inhibit the angiogenic process by blocking angiogenic factors or their receptors to prevent the growth of new tumor vessels. This type of therapy is represented by bevacizumab, a monoclonal antibody against vascular endothelial growth factor (VEGF), and several small molecular inhibitors of the VEGF receptor tyrosine kinase [26-30]. The goal of this strategy is to suppress the development of tumor neovasculature through blocking proliferation of endothelial cells. The second strategy is to kill the existing tumor endothelial cells. Compounds with this capability are referred to as “vascular disrupting agents” (VDAs). VDAs work by shutting down existing tumor vasculature, thereby depriving the tumor of adequate oxygen and nutrients, which leads to tumor ischemia and eventually tumor necrosis [11, 31]. There are two types of VDAs. One type of VDA is ligand-based and includes antibodies, peptides and growth factors that bind selectively to tumors but not to normal vessels by targeting tumor endothelial cells and occluding tumor vasculature. The other is a group of small molecules that include CA4P, ZD6126, AVE8062 and Oxi4503. These small molecules damage tumor endothelial cells by interfering with microtubule polymerization. Other small molecules include the flavonoid DMXAA, which induces localized release of TNFα or other cytokines from activated macrophages in the tumor tissue, which results in damage to tumor vessels. Currently several of these compounds are being actively pursued in pre-clinical tumor models and some have advanced into human clinical trials.
Colchicine-site microtubule interfering agents were initially developed as vascular disrupting agents, and this ability makes them attractive candidates for use in killing tumor endothelial cells as well. However, known colchicine-site microtubule interfering agents colchicine and vincristine require a near lethal dose to have such an effect and are thus not considered to be viable candidate compounds for clinical purposes. Combretastin A4 [8] is one example of a colchicine derivative that is currently being developed as a vascular disrupting agent [9-11]. In contrast to colchicine, the desired anti-vascular effect of combretastin A4 can be achieved at a dose only one tenth of the maximal tolerated dose in animal models. Clinical studies of combretastatin A4-phosphate, a water soluble pro-drug of combretastatin A4, have also confirmed its efficacy in decreasing tumor perfusion by dynamic contrast enhanced MRI and PET scan [12-14]. However, it was also observed that combretastin A4, at a dose higher than 68 mg/m2, exhibited cardiac toxicity and neurotoxicity, and also induced pain in the region of the tumor. These undesired side effects suggest that the use of combretastatin A4 for the treatment of cancer may be problematic. Another colchicine derivative, ZD6126, was also evaluated in phase I clinical trials [15]. Similar to combretastatin A4-P, cardiac toxicity (such as decreasing left ventricular ejection fraction) and dose limiting toxicity were observed. Thus, ZD6126 may also not be suitable for use in treating cancer.
There is an ongoing need to provide compounds that can be used to efficaciously treat cancer but that do not exhibit deleterious side effects.